1. Field of the Invention
This invention relates to methods and interfaces for facilitating testing or debugging an integrated circuit.
2. Description of Related Art
Debugging a problem in an integrated circuit (IC) is generally easier using a simulation of the IC rather than an actual IC because a simulation allows a circuit designer to probe the state of every signal and register in every module of the IC. The same design, when in the form of an IC, is more difficult to debug because most signals that the designer wants to observe are internal signals that are not connected to any I/O cells. However, significant debugging of actual integrated circuits is often required before an IC design is fully operational under a variety of system conditions.
One way to observe an internal signal for debugging is with a probe that can contact a pad or other structure within an IC, to measure a signal at the contact point. However, the small size of such structures make probing extremely difficult. Additionally, such structures are generally inaccessible when the IC is fully packaged. Focused ion beam (FIB) technology provides one method of bringing an internal signals out of an IC. FIB techniques "jump" an internal signal that a designer wants to observe, to an output line. A cost of jumping an internal signal to an output line is the loss of the function of the output signal normally on the line. The designer may be able to jump an internal signal to a non-critical output signal, but finding enough non-critical output signals becomes more difficult as the number of internal signals to be observed increases. Further, using FIB technology becomes tedious and expensive if the internal signal and the output signal are widely separated in the IC.
Accordingly, methods and circuits that permit observation of internal signals for debugging of an IC are desired. Further, such methods and circuits are sought which can be used on packaged IC and do not required jumping of signals, alteration of the IC, or precise positioning of probes.